New York citys 7.9 million people have a daily per capita consumption of 173 gallons of water. how many tons of sodium flouride would be requred per year to give this watr 1 part flourine per million parts water?

Close but not quite there.
I assume 1 part flourine per million parts really means 1 part flouride ION per million parts.

First, convert 1 part flouride ion to sodium flouride since the problem specifies using NaF.
1 part F x (molar mass NaF/molar mass F) = 1 x 41/19 = = ??. I have something like 2.2 or so and that will be tons if we keep all the other units in tons.

Next, convert 7.9 million people using 173 gallons water/day to gallons of water used per day.
173 gal/person x 7.3*10^106 people = gallons used per day.

Convert gallons of water used per day to cubic feet of water used per day using the density of water which is 62.4 lbs/cubic foot.

Convert lbs water used per day to tons per day using 2000 lbs = 1 ton

Now you have tons of water used per day and you want 2.2 tons sodium flouride in each 1,000,000 tons of water to make the 1 ppm F.